Walking with a hip joint prosthesis, particularly with an artificial leg secured to it, is difficult because three joints, namely the hip joint, the knee joint and the ankle joint have to be controlled without the prosthesis wearer being able to actively influence these joints. In particular, control of the movements of the hip joint is important for locomotion and for stability in the stance phase.
To ensure that the hip joint extends after a heel strike, the joint axes in conventional hip joint prostheses are arranged anterior to the joint axes of a natural hip joint. This concept is described in U.S. Pat. No. 3,090,964 A1, in which the proposed arrangement ensures that a maximum extension of the hip joint is adopted shortly after the heel strike.
U.S. Pat. No. 4,215,441 describes a leg prosthesis secured to a pelvic basket and having a spring which, during the stance phase, is biased by an axial force, e.g., the force resulting from the weight of the prosthesis user. If the prosthetic knee joint is flexed, the spring relaxes and causes a flexion in the hip joint and, consequently, a further flexion of the knee joint and a forward movement of the prosthetic foot. A similar mechanism is described in U.S. Pat. No. 4,051,558.
A leg prosthesis with a pneumatic energy storage unit is described in WO 01/17466 A2. Energy is stored during the extension movement in the hip joint, and the energy is released again during the subsequent flexion movement. The energy storage device comprises an electronically controlled piston/cylinder unit, the piston rod being able to telescope in order to limit the flexion of the hip joint in the swing phase. To ensure a stable and secure heel strike, a short step length is chosen.
Conventional devices for limiting the step length require manual actuation to permit substantial flexion, for example in order to sit down.
Additionally, it is not possible to achieve a truly natural gait pattern. A rapid extension of the hip joint after the heel strike is not a natural movement. The movement cannot be controlled by the patient, since it is difficult to balance on the free joint lying in front of the line of gravity. The natural lifting and swing-through of the contralateral leg during the extension of the artificial hip joint is not provided for in conventional hip joints. With a fixed step length, it is not possible to permit different walking speeds, since this requires different step lengths. In the case of a variable step length, which is dependent on the walking speed of the prosthesis user, a secure and stable heel strike must be ensured, without having to take into account the risk of a substantial and uncontrolled flexion of the hip. Damping of both the extension movement and also the flexion movement is not provided in conventional hip joints.